The present invention relates to a three-axis attitude control for a low-orbiting satellite.
Satellites in earth orbits frequently need to be controlled in all three attitude movement axes to fulfil the object of their missing. This is for example the case with most communications satellites, which have to be constantly aligned to the earth and in addition must not move in relation to the earth-oriented attitude movement axis.
In the case of earth oriented satellites, usually an earth sensor is used to measure the attitude, with the sensor providing attitude information concerning the roll axis and the pitch axis. To determine the yaw position in addition, usually further sensors are used, in the case of communications satellites, these are mainly sun sensors, magnetometers or speed gyroscopes. The use of an additional sensor has the disadvantage that it increases the costs for attitude control and the complexity of the measuring and control system. Furthermore, the use of an additional sun sensor in the case of low-orbiting satellites has the disadvantage that the earth shadow phase can extend across a region where the sun sensor does not provide any data.
However it is also possible to provide three-axis attitude movement stabilisation for an earth-oriented satellite in accordance with spin wheels in an earth sensor measuring in two axes. As is well known, this takes solace according to the so-called xe2x80x9cWhecon methodxe2x80x9d. In this method, a constant spin of the satellite perpendicular to the orbital plane is set by the spin wheels. The spin couples roll and yaw so that the attitude measurement in the roll axis carried out using the earth sensor, also includes attitude information about the yaw axis. By this implicitly present information, the movement of the yaw axis can also be controlled. The Whecon method was invented at the end of the sixties and since then has been in frequent use.
Operational safety of three-axis attitude determination using an earth sensor is affected by the possibility of failure or malfunction of the earth sensor. In the case of total failure of the earth sensor, i.e. if the earth sensor provides no information at all or incorrect information, conventional methods no longer provide the ability for three-axis stabilisation of the satellite position. In the case of limited function of the earth sensor as a result of failure or malfunction of individual sensor components, e.g. by solar or lunar irradiation in one or several detectors of the earth sensor, the earth sensor only provides single-axis attitude information. While this might be adequate for calculating three-axis attitude information, together with the two-axis attitude information of the additional sensor, e.g. a sun sensor or a magnetometer, this would only be the case if the information data of the two sensors has been obtained independently, which ie not usually the case. Through the geometrical attitude, the earth sensor provides single-axis attitude information which is already contained in the two-axis reading of the other sensor. in this case too, it is no longer possible to provide three-axis stabilisation for the satellite.
The requirements of reliability which exist for communication satellites can therefore necessitate a costly redundant design for an earth sensor.
As an alternative, three-axis star sensors can be used for three-axis attitude determination. This option is however very expensive and is therefore not used for communications satellitesxe2x80x94in particular in the case of satellite constellations using many satellites.
It is an object of the present invention to provide an economical and reliable three-axis attitude control for a low-orbiting satellite.
For three-axis attitude regulation the solution according to the present invention only requires a magnetometer measuring in two axes and spin wheels. For the application of the solution according to the present invention, the orbit of the satellite must not he too high, so that there is adequate field intensity of the magnetic field of the earth, and the orbital inclination must clearly be other than zero. These two preconditions are met in the case of low-orbiting satellite constellations used in communications and satellite navigation. Furthermore, satellites on sun-synchronous polar orbits and satellites on inclined eccentric orbits meet these preconditions in the partial region around the perigee.
When compared to an earth sensor measuring in two axes, the use of a magnetometer provides the advantage of increased reliability and an unlimited field of view.